1. Field of the Invention
The invention relates to a motor using a dynamic pressure air bearing as a radial bearing and as a thrust bearing by utilizing the magnetic attraction force between a magnet and a stator core.
2. Discussion of the Related Art
Radial bearings are used for preventing the center of rotation of a rotating device from being shifted from a predetermined position, even if a force is applied at a right angle to the axis of rotation.
Conventionally, motors have utilized a dynamic pressure air bearing as a radial bearing. FIG. 2 shows a conventional motor, which is used to rotate a polygonal mirror of a light deflecting apparatus. As shown in FIG. 2, a rotor portion of the motor is inserted to provide a space 15 between a rotation sleeve 3 and an axle 1. The axle 1 includes dynamic pressure generating grooves 1-1, and a balancer fitting groove 4 is provided in the rotation sleeve 3. The rotor further includes a yoke 5 and a magnet 6 fastened to the rotation sleeve 3 by press fitting or adhesive, and polygonal mirror 11 having a flange 12 and being fastened to the rotation sleeve 3 by screws 13.
The stator portion of the motor includes a housing 2, and the axle 1 is fastened to the housing 2 by press fitting or the like. A stator core 7 is fastened to the housing 2, a toroidal coil (not shown in the drawing figures) is coiled around the stator core 7, and a substrate 9 is hung by a stud 8 fastened to the stator core 7, with a magnetism detecting element 10 being fixed to the substrate 9.
The magnet 6 is a permanent magnet and a magnetic attraction force is generated between the magnet 6 and the facing stator core 7. The facing position of the magnet 6 and the stator core 7 is maintained in a certain position along the direction of the axis of the motor (thrust direction) by the magnetic attraction force.
That is, in FIG. 2, when the magnet 6 moves upward, a downward component appears in the magnetic attraction force to pull the magnet 6 down. On the other hand, when the magnet 6 moves downward, an upward component appears in the magnetic attraction force appears to pull the magnet 6 up. Thus, the magnet 6 and the stator core 7 face each other in the desired position along the direction of the axis, i.e., a magnetic thrust bearing is formed by the magnet 6 and the stator core 7.
The magnetism detecting element 10 may be a Hall element, for example, which detects the leakage flux of the magnet 6 and outputs detection signals to indicate whether an N-pole or S-pole has passed. The detection signals pass through a wiring printed on the substrate 9 and are transmitted to a controlling portion not shown in the figures. Based on the detection signal, the controlling portion determines the direction the electric current flowing in the toroidal coil, which is coiled around each part of the stator core 7. Consequently, a magnetic field occurs between the magnet 6 and the stator core 7 to keep the rotation sleeve 3 rotating.
By setting the dynamic pressure generating grooves 1-1, an air layer of high pressure is generated around the axle 1 (i.e. in the space 15) when the rotation sleeve 3 rotates. Thus, the rotation sleeve 3 is floatingly supported about the axle 1, that is, a dynamic pressure air bearing is formed.
Alternatively, the dynamic pressure generating grooves can be set on the inner wall of the rotation sleeve 3, though they are set on the circumference of the axle in the above example.
The air layer keeps the center of the rotation of the rotor portion in a certain position. For instance, in FIG. 2, if the rotation sleeve 3 is shifted rightward, the right side of the space 15 expands and then the pressure of that part of the space 15 becomes lower than before the rotation sleeve 3 shifted. On the other hand, because the left side of the space 15 becomes more narrow, the pressure in the left part of the space 15 becomes higher than before the rotation sleeve 3 shifted. If the relation of the pressure is as explained above, the rotation sleeve 3 will be moved leftward as a consequence of the pressure differential, and finally returned to the original position.
The polygonal mirror 11 has plural mirrors on the side faces of its circumference. The mirrors, for example, are irradiated by a light beam from a laser. As the polygonal mirror 11 rotates while the light beam irradiates a first mirror, a reflected light beam of the incident light beam gradually turns its direction, i.e., it is deflected.
The polygonal mirror 11 rotates further such that the light beam cannot irradiate the first mirror, then the second mirror comes around and is irradiated by the light beam. Light is then deflected by the second mirror. In this manner, the reflected light beams scan within a certain angular range, and the scanning speed depends on the rotating speed of the polygonal mirror 11.
However, the conventional motor described above has a problem in that the axis of the motor gets longer because the length of the axis direction of the magnet must be longer than that of the stator core.
FIG. 3 is an enlarged view showing the portions of the magnet 6 and the stator core 7 in a conventional motor. F is the gravitational force on the rotor portion, O.sub.6 is a center line of the magnet 6, O.sub.7 is a center line of the stator core 7, L is a gap between these center lines, H is a difference between the length of the magnet 6 and the stator core 7 in the axial direction, and .phi. is the magnetic flux.
Conventionally, sections of the facing surfaces of the magnet 6 and the stator core 7 make parallel lines. Magnetic flux .phi. entering the stator core 7 from the magnet 6 is the source of magnetic attraction force and creates the function of a thrust bearing.
If the magnet 6 and the stator core 7 are positioned such that their center lines O.sub.6 and O.sub.7 are colinear, magnetic flux .phi. is distributed in up-and-down symmetry with respect to the center lines. However, the center lines cannot be maintained in such a position. The reason is that the rotor portion including the magnet 6 is floating and the gravitational force F pulls downward, however, under the condition that the magnet 6 and the stator core 7 are located as described above, an upward balancing force countering the gravitational force F cannot be generated.
For the generation of a balancing force, the magnet 6 needs to descend so that magnetic flux density entering into the lower end of the stator core 7 from the lower end of the magnet 6 in the upper right direction becomes higher than that entering into the upper end of the stator core 7 from the magnet 6 in the lower right direction. The larger the difference between the magnetic flux density in the upper direction and that in the lower direction becomes, the larger the upward component of the magnetic attraction force. The magnet 6 stops descending when the upward component of magnetic attraction force is large enough to establish a balance with the gravitational force F. FIG. 3 is a view showing the state of the magnet 6 having descended as described above. As a consequence of the descent of the magnet 6, the gap L occurs between the center line O.sub.6 of the magnet 6 and the center line O.sub.7 of the stator core 7.
If the upper end of the magnet 6 falls lower than the upper end of the stator core 7, the magnetic flux .phi., which reaches a part of the toroidal coil around the stator core 7 that does not face the magnet 6, is reduced, and the generated torque becomes smaller. Thus, the length of the magnet 6 in the axial direction is made longer than that of the stator core 7 in order to prevent the upper end of the magnet 6 from dropping down below the upper end of the stator core 7 as a result of the gravitational force F. Therefore, in FIG. 3, H is a difference between the lengths of the magnet 6 and the stator core 7 in the axial direction Because the length of the magnet 6 must be longer than that of the stator core 7 in the axial direction, the axial length of the motor becomes longer. Empty internal space is also needed to accommodate the length of H in the axial direction.